1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter device, and more particularly to a surface acoustic wave filter device which includes internal reflection-type unidirectional transducers.
2. Description of the Related Art
Surface acoustic wave filter devices are known and practically used which generally include a piezoelectric substrate, and interdigital-type input-side and output-side transducers (IDTs) formed on the substrate for taking out a signal at a specific frequency range.
Conventional transducers for surface acoustic wave filter devices include transversal-type transducers wherein the surface acoustic wave excited by IDT electrodes propagates equally and bidirectionally on both sides of the transducer. It is known that the transversal-type transducers and surface acoustic wave filter devices using such transversal-type transducers are accompanied by an insertion loss (a so-called "6dB loss"). Thus, in order to decrease the insertion loss of the conventional transversal-type transducers as noted above, various types of unidirectional transducers for surface acoustic wave filter devices have been proposed, wherein the surface acoustic wave propagates unidirectionally in normal propagation direction.
Unidirectional transducers according to such proposals can be roughly classified into three types, i.e., a three-phase-type transducer, a group-types transducer and an internal reflection-type transducer, which will be explained below.
The three-phase-type unidirectional transducer includes three kinds of IDT electrode fingers which are respectively applied with input signals having a phase difference of 0.degree., 120.degree., and 240.degree.. The three-phase-type transducer is capable of maintaining the desired unidirectional property of the surface acoustic wave propagation over a wide frequency range. On the other hand, however, the three-phase-type transducer requires complicated phase shifters and also a particular arrangement wherein the electrode finger extending from one of three bus bars must be bridged over another bus bar. Therefore, the three-phase-type transducer is generally difficult to manufacture and, hence, costly.
The group-type unidirectional transducer includes meander lines as ground lines which are arranged between the IDT electrode fingers in a meandering manner. The group-type transducer is disadvantageous in that a 90.degree. phase shifter (e.g., a coil) is required and the total length of the meander line becomes long, which results in a higher insertion loss of the filter device due to an ohmic loss.
The internal reflection-type unidirectional transducer may be provided with aluminum IDT electrode fingers which are combined with electrode fingers of a high density metal, such as gold, so that the distance between the center of excitation area and the center of reflection area of the surface acoustic wave is made .lambda./8 with reference to the wavelength .lambda. of a fundamental surface acoustic wave which has been excited. While the internal reflection-type transducer does not require a phase shifter, it had been necessary to perform not only vapor evaporation for the aluminum electrode fingers, but also an additional vapor evaporation for the gold electrode fingers to be paired with the aluminum electrode finger. Hence there had been a disadvantage that the manufacturing process becomes complicated.
One proposal has been made with respect to the internal reflection-type unidirectional transducer, which is to provide floating electrodes instead of the gold electrode fingers so that all the electrodes can be formed by a single processing step. Another proposal related to the internal reflection-type unidirectional transducer is to combine an electrode of a width .lambda./8 with another electrode of a width 3.lambda./8. In these instances, there arises a tendency of a slight increase in the insertion loss due to a partial energy leakage of the surface acoustic wave in a reverse direction, i.e., a direction opposite to the propagation direction. Besides, the internal reflection-type unidirectional transducer is generally unsuitable for substrates with a small piezoelectric property, such as quartz, since the piezoelectricity of the substrate plays an important role in the reflection. However, there are instances wherein it is desirable to use quartz or the like substrate materials with a small piezoelectric property.
Apart from the above-mentioned problems, another important factor associated with surface acoustic wave filter devices is to realize an improved manufacturing yield of the unidirectional transducers. More particularly, the unidirectional transducer includes a positive electrode and a negative electrode, each comprising a plurality of electrode fingers which may be formed by a photo-lithographic process after formation of an aluminum layer on the substrate. For each of the positive and negative electrodes, the electrode fingers are spaced from each other by a center distance which corresponds to the wavelength .lambda. of the fundamental surface acoustic wave. The electrode fingers of the positive electrode and the electrode fingers of the negative electrode are arranged alternately with each other, with a center distance of .lambda./2. In this instance, for example, the width of each electrode finger is set as .lambda./12. Thus, as for a filter device including a ST-cut quartz substrate and having a center frequency of 240 MHz, for example, the electrode finger width of .lambda./12 is as fine as approximately 1.1 .mu.m, and such a fine width often results in practical difficulties to achieve a satisfactory manufacturing yield.